title: Internal sodium ions and water molecules in guanine quadruplexes: magnetic relaxation dispersion studies of [d(G3T4G3)]2 and [d(G4T4G4)]2.
abstract: The structural stability of guanine quadruplexes depends critically on an unusual configuration of dehydrated Na (+) or K (+) ions, closely spaced along the central axis of the quadruplex. Crystallography and NMR spectroscopy indicate that these internal ions can be located between the G-quartet planes as well as in the thymine loops, but the precise ion coordination has been firmly established in only a few cases. Here, we examine the bimolecular diagonal-looped foldback quadruplexes [d(G 3T 4G 3)] 2 (Q3) and [d(G 4T 4G 4)] 2 (Q4) by (2)H, (17)O, and (23)Na magnetic relaxation dispersion (MRD). The MRD data indicate that both quadruplexes contain Na (+) ions between the T 4 loops and the terminal G-quartets and that these ions have one water ligand. These ions exchange with external ions on a time scale of 10-60 mus at 27 degrees C, while their highly ordered water ligands have residence times in the range 10 (-8)-10 (-6) s. The MRD data indicate that Q4 contains three Na (+) ions in the stem sites, in agreement with previous solid-state (23)Na NMR findings but contrary to the only crystal structure of this quadruplex. For Q3, the MRD data suggest a less symmetric coordination of the two stem ions. In both quadruplexes, the stem ions have residence times of 0.6-1.0 ms at 27 degrees C. The equilibrium constant for Na (+) --> K (+) exchange is approximately 4 for both loop and stem sites in Q3, in agreement with previous (1)H NMR findings.
authors.affiliation: Department of Biophysical Chemistry, Center for Molecular Protein Science, Lund University, SE-22100 Lund, Sweden. Karim.Snoussi@uclouvain.be
authors.forename: Karim
authors.initials: K
authors.lastname: Snoussi
authors.forename: Bertil
authors.initials: B
authors.lastname: Halle
mesh_terms: D002414:Cations, Monovalent / Q000737:chemistry; D018360:Crystallography, X-Ray; D006147:Guanine / Q000737:chemistry*; D009682:Magnetic Resonance Spectroscopy; D008958:Models, Molecular; D009690:Nucleic Acid Conformation; D009841:Oligonucleotides / Q000737:chemistry*; D011188:Potassium / Q000737:chemistry; D012964:Sodium / Q000737:chemistry*
chemical_list: D002414:Cations, Monovalent; D009841:Oligonucleotides; D006147:Guanine; D012964:Sodium; D011188:Potassium